In one aspect, a Multiple Input Multiple Output (“MIMO” for short) technology can provide transmit (receive) beamforming, thereby effectively improving transmit (receive) power and effectively improving reliability of a communications system. In another aspect, the MIMO technology can generate an extra spatial degree of freedom, thereby improving a system throughput manyfold and effectively increasing a rate of the communications system. Due to these advantages of the MIMO technology, the MIMO technology is a key technology in the 802.11n standard protocol. The MIMO technology in the 802.11n standard includes space time coding, beamforming, antenna selection, coherence combination, and spatial multiplexing, and supports simultaneous transmission of four data flows. Therefore, compared with the 802.11a/b/g standard, the 802.11n standard significantly improves the system throughput, and a maximum transmission rate reaches 600 Mbit/s theoretically.
To further improve the system throughput, the IEEE Union is drafting and revising the 802.11ac standard following the 802.11n standard. The standard is dedicated to a frequency band of 5 GHz, a bandwidth increases from 40 Mbit/s in the original 802.11n standard to 80 Mbit/s, and even reaches 160 Mbit/s. In addition, the 802.11ac standard supports a higher-order modulation scheme—256 quadrature amplitude modulation (Quadrature Amplitude Modulation, “QAM modulation” for short). To further improve an MIMO degree of freedom, the 802.11ac standard supports simultaneous transmission of a maximum of eight flows. Considering an asymmetrical quantity of link antennas, a downlink multi-user MIMO (“MU-MIMO” for short) technology is introduced in the 802.11ac standard, thereby effectively utilizing the MIMO degree of freedom. Because of all these advanced technologies, a throughput in the 802.11ac standard exceeds 1 Gbps.
Actually, the downlink MU-MIMO technology is a spatial multiplexing based one-to-many transmission technology. By using the downlink MU-MIMO technology, a transmit end may simultaneously transmit data to multiple receive ends or multiple users. When MIMO transmission is being performed, data of each flow needs to be aligned. When data is transmitted to multiple users, although the data transmitted to the users is not the same, the data still needs to be aligned by means of zero filling, where zero filling is zero padding, or is referred to as bit trailing. In a current technology, data of different flows is aligned by means of zero filling at a media access control (Media Access Control, “MAC” for short) layer and a physical (Physical, “PHY” for short) layer.
To further improve a throughput of a wireless local area network, and resolve problems of severe interference in a dense scenario and a low throughput rate, an uplink MU-MIMO technology and an orthogonal frequency division multiple access (Orthogonal Frequency Division Multiple Access, “OFDMA” for short) technology are to be further considered in a future Wireless Fidelity (Wireless Fidelity, “WiFi” for short) system or WiFi standard. Same as MU-MIMO, an OFDMA system requires data to be aligned by means of zero padding in a frequency domain. When both OFDMA and MU-MIMO are used, that is, MU-MIMO is performed on an OFDMA subband, zero padding needs to be first performed between different flows for data alignment, and then data alignment is performed on different subbands.
When MIMO transmission and MU-MIMO transmission are performed on an OFDMA subband, there is a great length difference between data transmitted between subbands and between flows, and more zeros need to be padded for data alignment. In this way, a waste of frequency band resources is caused; in addition, zero padding brings extra overheads. This is undesirable.
Therefore, in a data transmission process, how to reduce extra overheads caused by zero padding for data alignment and improve frequency band resource utilization is an urgent problem to be resolved.